Because of politics. It's getting more and more difficult and expensive to get anything done. There's a $3 billion hand drill there already, but it didn't drill for almost a year.
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LocalFluffJun 1 '14 at 0:00

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The InSight lander, due to be launched in 2016, includes a probe designed to burrow up to 5 meters below the surface -- but it's designed to measure heat flow, not water.
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Keith ThompsonJun 2 '14 at 21:59

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Sending anything to Mars is expensive. Sending a machine capable of drilling to the required depths to find subsurface water would be prohibitively expensive with current rocket payload limits.

There are also far more accessible sources of water on the surface - lakes of ice in craters, water rich minerals, and even an equatorial frozen sea .

There is also water at the surface of Mars on the poles, but the conditions here are much harsher than nearer the equator, and there is less sunlight, reducing the capability of any solar powered rover.

The subsurface water is not necessarily that far below the surface, but it is also not a torrent; more likely trapped as ice particles in between the rock or in secondary minerals.

Does the thin atmosphere of Mars really block so much sunlight that there is any practically relevance difference between the equator and the poles, during polar summer? Given of course that the solar panel is directed towards the Sun, which has not been the design for rovers thus far.
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LocalFluffJan 13 at 14:57

This link gives an equation to calculate the extinction of atmospheric light given zenith angle. You can see from the chart further down the page titled average atmospheric extinction that once you pass an angle of ~70 degrees from the zenith that the extinction exceeds 100%. Mars has a tilt similar to earths (~ 25 degrees), so a rover at the pole wouldn't have to point its solar panels at the horizon to get significant sunlight, however it would be in the dark for half of the martian year, which is about twice as long as an Earth year.
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polyphantJan 15 at 13:20